IF YOU CONTRIBUTE in any way to the D.C. Treasury, you may wish to know about a gift that you helped buy last week: it was for Arrington Dixon, who in September had fewer supporters at the polls than did David Clarke, and thus is bidding farewell to city hall and the job of council chairman. As a goodbye present to help him through this transition to private life, the council members took a moment out of their final meeting of the year to vote Mr. Dixon the sum of $25,000.

What a generous custom, based apparently on a similar gesture at the time the last chairman departed. True, there are expenses involved in leaving government service, and surely the council must have in mind a scale of farewell funds to cover each grade of employee upon his or her termination. And if things hold true to form, the amounts should be seasonally adjusted and tied to the consumer price index as projected to fiscal 1987.

Certainly those expenses that a top city official does incur strictly as a result of leaving office -- official correspondence comes to mind -- should be covered, not under a lump sum but through an expense account with a cap, determined for all such job-holders, not by his colleagues on a case-by-case vote.

Public service at the highest levels involves sacrifices as well as rewards, but any financial rewards should be spelled out up front and clearly. We wish Mr. Dixon well, of course; but his colleagues have made more of it than they should have. The mayor should put a stop to this--and find a more acceptable way to reimburse Mr. Dixon for approved expenses.